Various types of membrane or pressure sensitive switches are known in the art. Such switches typically comprise a pair of resilient laminae, typically made of a polyester material. A typical manner in which the switch is formed is one wherein a silver conductive ink circuit is screen printed onto a side of one lamina facing the opposite lamina. A pad circuit formed from silver conductive ink is screen printed onto the other lamina on a side facing the first lamina and at a location such that the circuit and pad circuit will be superimposed upon one another when the two laminae are in engagement.
A spacer sheet is interposed between the laminae and has a window cut out at the location of the circuit network and pad circuit. The spacer can also be manufactured of a polyester material and can be secured to the laminae by a pressure sensitive material.
With such an arrangement and cooperation of the various components of the switch, the switch is normally in a position wherein the circuit network and pad circuit are out of engagement. Because of the flexible nature of the laminae, however, when tactile or other pressure is applied to one of the laminae at the location of the window, the conductive ink screens printed on the laminae will be made to engage one another to complete a circuit. Completion of a circuit in this manner can, in turn, effect a particular function depending upon the application to which the switch is put.
Numerous applications exist for such membrane switches. Illustrative of some of the applications to which such switches can be put are functional controls for microwave ovens, dispensing switches for vending machines, keyboards for computers, and innumerable other specific applications.
In some of such applications, it is desirable, for various reasons, to utilize a membrane switch having a large activation area. The requirement for such a switch, however, presents unique problems. Self actuation can occur if the window in the spacer between the laminae is too large. The ease with which inadvertent actuation occurs as the window is enlarged is, of course, a function of the thickness of the spacer. Since spacer thickness is one way in which a switch is made more course, enlargement of the window, even if not effecting undesired actuation, will make the switch more sensitive. It can be seen, therefore, that, as the window is enlarged, the switch will not operate in the manner in which it ws originally designed to operate.
It has been found that, when using a spacer having a seven mil thickness, if the smaller dimension of the window is made to exceed three quarters of an inch, inadvertent closing of the switch can occur. If the same switch is provided with a spacer having a thickness of, for example, ten mils in order to provide for more course actuation, as the smaller dimension of the window is made to exceed three quarters of an inch, although the switch might not immediately become susceptible to inadvertent closing, it will become more sensitive than what is desired.
In some applications which have previously been discussed (i.e., vending machine actuation switches) it is desirable to have a switch having a smaller dimension significantly in excess of three quarters of an inch. The invention of the present application is a membrane switch which solves the problems of the prior art as discussed above. It can have virtually unlimited dimensions without either being more susceptible to inadvertent actuation or altering the sensitivity of the switch.